1. Field of the Invention
The present invention relates to a semiconductor laser device having a double heterostructure, in which plural semiconductor layers including an active layer are formed on a substrate, and more particularly to such a semiconductor laser device emitting a high power laser beam.
2. Description of Related Art
A semiconductor laser device is used for measuring a distance, for example, in a vision system of a robot and in a radar system. A laser device emits a laser beam toward an object and receives a reflected beam therefrom. A distance between the object and the laser device is measured based on a delay time of the reflected beam which depends on the distance. Since a measurable distance by a laser device depends on its power, a high power laser is necessary to measure a long distance. For example, to measure a distance of 100 m between two cars, a pulse driven laser device having an output of several tens watts is required. To obtain the light output of several tens watts, the laser device has to be driven with pulse current of several tens amperes.
A semiconductor laser device shown in FIGS. 12A and 12B is known as a high power device. This laser device includes an active layer having a multi-quantum-well structure and optical guide layers and clad layers disposed on both sides of the active layer. This structure is proposed to effectively confine light and current. In FIG. 12A, a depth from a top surface is shown on the abscissa and an aluminum-mixing ratio in the layers is shown on the ordinate. On an n-GaAs (n-type gallium arsenide) substrate 102, a first clad layer 103, a first optical guide layer 104, an active layer 105, a second optical guide layer 106, a second clad layer 107 and a p-GaAs (p-type gallium arsenide) layer 108 are laminated in this order. The active layer 105 has a multi-quantum-well structure in which layers made of an AlGaAs-based (aluminum-gallium-arsenide) material and layers made of a GaAs-based material are alternately laminated. Each of such layers in the active layer 105 is made sufficiently thin to a level of an wave-length of de Broglie of an electron and a hole, or less. A total thickness of the active layer 105 is made around 0.1 .mu.m to effectively confine electric current therein. The clad layers 103, 107 and optical guide layers 104, 106 are made of an AlGaAs-based material in which an Al-mixing ratio (a ratio of Al in AlGa) is properly selected so that each layer performs a desired function.
In FIG. 12B, the depth from the top surface is shown on the abscissa and a refractive index is shown on the ordinate. A band gap of each layer depends on the Al-mixing ratio, and a refractive index thereof depends on the band gap. Therefore, each layer has its refractive index as shown in FIG. 12B. Thus, a SCH structure (separate confinement heterostructure) having a desired refractive index distribution is obtained. Among layers 103, 104, 105, 106 and 107, the active layer 105 has the highest refractive index, the optical guide layers 104, 106 formed on both sides of the active layer 105 have an intermediate refractive index, and the clad layers 103, 107 have the lowest refractive index. Light generated in the active layer 105 is amplified in a region of the active layer 105 and optical guide layers 104, 106 and is distributed as shown by a dotted line in FIG. 12B. Since the light density is distributed, energy concentration to the active layer 105 is alleviated.
A semiconductor laser device having a so-called GRIN-SCH structure (graded index separate confinement heterostructure) is shown in U.S. Pat. No. 4,905,246. In this device, the refractive index of the optical guide layers formed on both sides of the active layer is varied continuously between the active layer and the clad layer. Energy concentration in the active layer is alleviated by distributing light density to both optical guide layers.
Though the light density is distributed in those conventional devices, a peak of the light density is still in the active layer which is made very thin and overlaps with a peak of carriers (current) injected into the active layer. Therefore, the energy concentration in the active layer is not sufficiently reduced. The energy concentration causes dislocation in a crystal of the active layer, which in turn deteriorates the active layer during a long time operation. This results in output power decrease and shortening a life of the device. It is important to avoid such a energy concentration especially in a high power laser device, such as a pulse driven device having an output of several tens watts.